Help with Marshall mg100

[banjoboybc]

HI, I replaced the 7293 chip and powered up amp it looked good, then I noticed the fan was bad so I replaced it and now when i power up the fuse blows. unplug the amp from main board and it powers up ok, what is going on with this amp please help. thanks

[Jazz P Bass]

The back plane of the 7293 IC is at B-Vdc potential.
The back plane is attached to the heatsink.
The fan is attached to the heatsink by (4) screws.
But...
From the factory, Marshall supplied (4) white plastic "isolators" at the fan screw attachment points.
These isolate all of the B- stuff from the chassis, which is at ground potential.
Chances are good that one or more is missing or damaged.
Before you power up the amp, check continuity from the heat sink to the chassis.
It should read open circuit.

[banjoboybc]

thanks i will check it out. and thank you very much

[Ronsonic]

Jazz P nailed it. You'll notice that the mounting on that fan and heatsink is a bit weird and awkward.

Good move replacing the fan. And for anyone looking for info on these amps, always replace that fan if it so much as makes a peep. That fan is essential to keeping the output module alive and it is a fragile cheap fan.